Electrically controlled optical elements and method

ABSTRACT

The effective focal length of an optical system can be electronically controlled using switchable wave plates in conjunction with polarized light.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/321,430 filed Jul. 1, 2014, now U.S. Pat. No. 9,223,155 which is acontinuation of U.S. patent application Ser. No. 13/221,758 filed Aug.30, 2011, now U.S. Pat. No. 8,767,284 which is a continuation of U.S.patent application Ser. No. 12/561,156 filed Sep. 16, 2009, now U.S.Pat. No. 8,009,349 which is a continuation of U.S. patent applicationSer. No. 11/086,188 filed Mar. 21, 2005, now abandoned, which claimspriority to U.S. Provisional Application 60/554,870, filed Mar. 22,2004.

BACKGROUND

It is desirable in many optical systems to be able to dynamically changethe focal length of or effective airspaces in an optical layout. Forexample, in a camera, it is often advantageous to have a zoom lens thatis capable of altering its focal length in order to change themagnification of an image. In other optical systems, such as viewfindersfor near-to-eye virtual reality displays, it is beneficial to have aviewfinder that can quickly switch from creating a wide-angle, immersiveimage to displaying a narrow angle, high resolution image.Implementation of zoom, angle adjustment, and other focal-lengthdependent dynamic optical alterations typically require mechanicaladjustment—an element or optics group is moved in relation to others andthe overall focal length of the system is adjusted.

The mechanical adjustment of such systems requires the motion of anelement and can thus be adjusted only as quickly as the elements can bemoved. The mechanism for such adjustment requires some sort of motor ifa computer is to control the adjustment. The attendant size of theadjustment mechanism can be difficult to incorporate into small camerassuch as those in mobile telephones or virtual reality displays whereweight is often an important factor. Mechanical adjusters are prone tobreakage and must be kept clean in order to function well, necessitatingthe delicate treatment of zoom camera lenses, microscopes, and otherequipment optical equipment with mechanically adjusted focal lengths.

Accordingly, it is desirable to provide electrically controlled opticalelements and an associated method. The electrically controlled opticalelements afford a method for altering an optical system's configurationwithout physically moving any of the optics, allowing a compact,durable, quickly adjustable optical package.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates in side plan view an application ofelectrically controlled optical elements in accordance with anembodiment of the invention.

FIG. 2 schematically illustrates in perspective view an application ofelectrically controlled optical elements in accordance with anembodiment of the invention.

FIG. 3 schematically illustrates in plan view the operation ofelectrically controlled optical elements in accordance with anembodiment of the invention.

FIG. 4 schematically illustrates in plan view the operation ofelectrically controlled optical elements in accordance with anembodiment of the invention.

FIGS. 5a and 5b schematically illustrate in cross sectional views aprior art zoom lens in a first and second zoom state, respectively.

FIGS. 6a and 6b schematically illustrate in cross sectional views a zoomlens that employs electrically controlled optical elements in a firstand second zoom state, respectively.

SUMMARY OF THE INVENTIONS

Electrically controlled optical elements manipulate the polarization oflight in order to vary the effective focal length of an opticalinstrument. This invention utilizes an element, such as a switchableferro-electric half wave plate mirror, that is capable of variablyswitching between the polarizations of light it passes through theoptical assembly. When the switchable element passes light of one kindof polarization, the light travels through the optical elementsnormally. When the switchable element passes light of a differentpolarization, however, the light is reflected twice within the assembly,resulting in a longer effective focal length than before. By usingpolarization to control the path of light within the optical elementsand folding the light path, it is possible to create a compact zoom lensor a near to eye display with multiple fields of view without any movingparts.

An electrically controlled optical system for controlling the focallength of an optical system on a light path includes a switchable waveplate having two or more states, the switchable wave plate receivinglight input to the light path, and an electrical means for controllingthe state of the switchable wave plate, a half-silvered mirror receivinglight from the switchable wave plate, a polarization dependent mirrorreceiving light from the half-silvered mirror, and one or more opticalelements.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1 and 2 schematically illustrate electrically controlled opticalelements as applied in a near-to-eye application, such as one found in avirtual reality head-mounted display or viewfinder. Viewfinder 10consists of electrically controlled optical elements 12 as well asdiffusion screen 14 and viewing optics 16. The components of viewfinder10 are contained within an enclosure, such as an aluminum housing (notillustrated). As illustrated, light rays 18 exit a display, such as anLCD 17, enter viewfinder 10 from the right, and exit the viewfinder fromthe left into a viewer's eyeball 20.

Electrically controlled optical elements 12 consist of linear polarizer22, quarter wave plate 24, ferro-electric switchable half wave plate 26,control electronics 27, half silvered mirror 28, lens 30, quarter waveplate 32, reflective polarizer 34, and linear polarizer 36. The elementsof viewfinder 10 are positioned so that they are substantially parallelto one another. The distances shown in FIGS. 1 and 2 are purely forillustrative purposes; those of skill in the art will recognize that theindividual components of the system could be separated by differentdistances illustrated in FIGS. 1 and 2 without departing from the spiritof the invention and optics 16 and 30 would be composed of one or morelenses and/or air spaces.

Linear polarizer 22 may be oriented as an “S” or a “P” linear polarizer;in this illustrative embodiment, polarizer 22 is an “S” polarizer andpasses light with “S” polarization. Quarter wave plate 24 circularlypolarizes the linearly polarized light that passes through linearpolarizer 22. The linearly polarized light that transmits through linearpolarizer 22 is thus circularly polarized with an “L” handedness.

Ferro-electric switchable half wave plate 26 may be, for example, aswitchable half wave plate made by Displaytech or others. Theferro-electric switchable half wave plate is oriented to pass thecircularly polarized light that exits quarter wave plate 24. The lighttransmitted can be controlled to be a left or right handed by switchingthe switchable wave plate. The wave plate and switchable element couldbe a single switchable plate with suitable switchable retardationperformance, such that the handedness of the light's circularpolarization, is controlled by the state of the electrical signalsreceived from control electronics 27. For the purposes of illustration,if ferro-electric switchable half wave plate 26 is “on”, it reverses thehandedness of the light's polarization that passes through it, and ifthe ferro-electric switchable half wave plate is “off”, it does notchange the handedness of the light's circular polarization that passesthrough it. Note that the terms “on” and “off” as applied to theferro-electric switchable half wave plate are generic terms, asswitching between the “on” and “off” states on the ferro-electricswitchable half wave plate may involve reversing a voltage applied tothe ferro-electric switchable half wave plate or the like.

Those of skill in the art will recognize that ferro-electric switchablehalf wave plate may be replaced by an LCD or the like that can similarlyperform the task of selectably altering the circular polarization oflight without departing from the spirit of the invention, and that thiscould be done in an array, not just in a single large shutter.

Control electronics 27 are configured to electrically govern whetherferro-electric switchable half waveplate is in an “on” or “off” state byapplying signals to the ferro-electric switchable half wave plate. Thecontrol electronics may include, for example, a microchip, a centralprocessing unit, or the like. Signals to the ferro-electric switchablehalf wave plate may include an applied voltage, an alternating current,or the like.

Half silvered mirror 28 is designed to reflect a percentage of lightthat passes through ferro-electric switchable half wave plate 26 andtransmit the remainder of the light. The percentage of light that thehalf silvered mirror reflects versus transmits may be tailored to meetparticular design specifications by altering the coatings on the halfsilvered mirror or the like. For the purposes of illustration, halfsilvered mirror 28 is designed to reflect fifty percent of the lightthat passes through ferro-electric switchable half wave plate 26 andtransmit the other fifty percent of the light. Surface 29 and/or surface31 may be coated in order to reflect fifty percent of the light andtransmit fifty percent of the light (losses would alter this a bit).

Lens 30 is a lens and refracts the light transmitted through halfsilvered mirror 28. In the preferred embodiment of the invention lens 30is a glass lens that does not affect the polarization of the light thatpasses through it. While the illustrated lens in FIGS. 1 and 2 is acircular, double-convex lens, those of skill in the art will recognizethat other possible lens types, such as a fresnel lens, a lens group, orthe like, also find application in the present invention.

Quarter wave plate 32 linearly polarizes the circularly polarized lightthat passes through lens 30. Thus, light that is circularly polarizedwith an “L” handedness becomes “S” linearly polarized and light that iscircularly polarized with an “R” handedness becomes “P” linearlypolarized when passing through quarter wave plate 32.

Reflective polarizer 34 reflects linear polarization of one type andtransmits the linear polarization of the other type. Reflectivepolarizer 34 may be oriented as either an “S” or a “P” polarizer,however, as configured in the illustrated embodiments, reflectivepolarizer 34 reflects linear polarizations of the same type that linearpolarizer 22 passes. Thus, for the purposes of illustration, reflectivepolarizer 34 is a “P” polarizer that reflects “S” polarized light andtransmits “P” polarized light. Those of skill in the art will recognizethat quarter wave plate 32 and reflective polarizer 34 need not beseparate elements but may be made integrally, either by bonding the twotogether or manufacturing them together as one element, withoutdeparting from the spirit of the invention.

Linear polarizer 36 may be oriented as either an “S” or a “P” polarizer.Those of skill in the art will recognize that it is possible to buildthe present invention without linear polarizer 36; however, in thepreferred embodiment of the invention linear polarizer 36 is included.

Diffuser 14 of viewfinder 10 acts as a diffusion screen on to whichimages from the display are projected by electrically controlled opticalelements 12. The angles at which diffuser 14 disperses light may beselected depending on the optical requirements of the system. Those ofskill in the art will recognize that it is possible to build aviewfinder without diffuser 14 without departing from the spirit of theinvention.

Lens 16 is a lens and refracts the light that passes through diffuser 14and focuses the light rays for eyeball 20. While the illustrated lens inFIGS. 1 and 2 is a circular, double-convex lens, those of skill in theart will recognize that other possible lens types, such as a fresnellens, a lens group, or the like, also find application in the presentinvention.

In an additional embodiment of the invention (not illustrated), lens 30is removed from electrically controlled optical elements 12 and only airoccupies the space between half silvered mirror 28 and quarter waveplate 32. Certain applications, like inexpensive zoom lenses, may notrequire the additional expense of a lens or may necessitate the removalof the lens due to packaging concerns. Those of skill in the art willrecognize that it is possible to fill the volume of space between thehalf silvered mirror and the quarter wave plate with a material thatpossesses an index of refraction different than air, such as water,without departing from the spirit of the invention.

In an additional embodiment of the invention (not illustrated), quarterwave plate 32 is located between lens 30 and half silvered mirror 28. Itmay also be bonded to the half silvered mirror to make the two a single,integral element.

In an additional embodiment of the invention (not illustrated),ferro-electric switchable half wave plate 26 and quarter wave plate 24are combined into a single electro-optic shutter.

In an additional embodiment of the invention (not illustrated), aferro-electric switchable quarter wave plate is used in combination witha static half wave plate in place of quarter wave plate 24 andferro-electric switchable half wave plate 26. Alternatively, wave plates24 and 26 could be a single electro-optical element.

Illustrative Operation of Viewfinder Utilizing Electrically ControlledOptical Elements

FIG. 3 shows in plan view the operation of viewfinder 10 withferro-electric switchable half wave plate “on”. With continuingreference to FIGS. 1 and 2, unpolarized light rays 18 exit the display17 and enter viewfinder 10. Light rays 18 pass through linear filter 22and become “S” polarized (as represented by light rays 18(a)) beforepassing through quarter wave plate 24 where the “S” polarized light raysbecome circularly polarized with an “L” handedness. Light rays 18(b)then transmit through ferro-electric switchable half wave plate 26.Because the ferro-electric switchable half wave plate is turned “on” bycontrol electronics 27, the handedness of the circular polarization thelight rays is reversed so that light rays 18(c) exiting ferro-electricswitchable half wave plate have an “R” handedness. Next, “R” circularlypolarized light rays 18(c) transmit to half silvered mirror 28 wherehalf the light rays (light rays 18(d)) reflect off the half silveredmirror and half the light rays (light rays 18(e)) transmit through thehalf silvered mirror. Light rays 18(e) enter lens 30, which refracts thelight rays on to quarter wave plate 32. When light rays 18(f) exit thequarter wave plate, their polarization state has been switched fromcircular “R” to linear “P”. Light rays 18(f) next intersect reflectivepolarizer 34. Because light rays 18(f) are “P” polarized and reflectivepolarizer 34 is a “P” polarizer, the light rays pass through thereflective polarizer unchanged before passing through linear polarizer36, which also does not change the light rays' polarization. Light rays18(f) form an image on diffuser 14; this image has a height A (forexample, 4 inches). Lens 16 collects light rays 18(g) and focuses themon eyeball 20. Based on the distance B between eyeball 20 and lens 16(for example, one inch), the focal length of lens 16, and the height Aof the image formed by diffuser 14, viewfinder 10 in the presentconfiguration has a viewing angle θ.

FIG. 4 shows in plan view the operation of viewfinder 10 with ferroelectric switchable half wave plate “off”. With continuing reference toFIGS. 1 and 2, unpolarized light rays 18 exit the display (notillustrated) and enter viewfinder 10. Light rays 18 pass through linearfilter 22 and become “S” polarized (as represented by light rays 18(i))before passing through quarter wave plate 24 where the “S” polarizedlight rays are circularly polarized with an “L” handedness. Light rays18(j) then transmit through ferro-electric switchable half wave plate26. Because the ferro-electric switchable half wave plate is “off”, the“L” handedness of the circular polarization of light rays 18(j) remainsunchanged. Next, light rays 18(j) transmit to half silvered mirror 28where half the light rays (light rays 18(k)) reflect off the halfsilvered mirror and half the light rays (light rays 18(l)) transmitthrough the half silvered mirror. Light rays 18(l) enter lens 30, whichrefracts the light rays on to quarter wave plate 32. The quarter waveplate changes the polarization state of the light rays from circular “L”to linear “S”, so light rays 18(n) that exit the quarter wave plate are“S” polarized. Light rays 18(n) next intersect reflective polarizer 34.Because light rays 18(n) are “S” polarized and reflective polarizer 34is a “P” polarizer, light rays 18(n) do not transmit through thereflective polarizer but rather reflect off it. The light rays are still“S” polarized. Light rays 18(n) pass through quarter wave plate 32 againand light rays 18(p) that exit the quarter wave plate have circularpolarization with “L” handedness. Lens 30 collects light rays 18(p) andrefracts the light rays on to half silvered mirror 28. Half of lightrays 18(p) reflect off half silvered mirror 28 (light rays 18(r)) andhalf the light rays transmit through the half silvered mirror (lightrays 18(s)). Reflecting off half silvered mirror 28 causes thehandedness of the light rays' circular polarization to reverse; lightrays 18(r) now are circularly polarized with an “R” handedness. Lens 30collects light rays 18(r) and refracts the light rays on to quarter waveplate 32. The quarter wave plate changes the polarization of the lightrays from circular “R” to linear “P”. Light rays 18(u) next intersectreflective polarizer 34. Because light rays 18(u) are now “P” polarizedand reflective polarizer 34 is a “P” polarizer, the light rays passthrough the reflective polarizer unchanged before passing through linearpolarizer 36, which also does not change the light rays' polarization.Light rays 18(u) form an image on diffuser 14; this image has a height C(for example, 7 inches). Lens 16 collects light rays 18(v) and focusesthem on eyeball 20. Based on the distance B between eyeball 20 and lens16 (for example, one inch), the focal length of lens 16, and the heightC of the image formed by diffuser 14, viewfinder 10 in the presentconfiguration has a viewing angle β.

As it is configured in FIGS. 1-4, the electrically controlled opticalelements in viewfinder 10 produce a larger image, and thus a more highlymagnified image, when the ferro-electric switchable half wave plate isturned “off”. The viewing angle, β, when the ferro-electric switchablehalf wave plate is “off” is also larger than the viewing angle, θ, whenthe ferro-electric switchable half wave plate is “on”. Note that theeffective focal length of the electrically controlled optical elementsincreases when the ferro-electric switchable half wave plate is “on”because light rays must traverse the distance between the reflectivepolarizer and the half silvered mirror three times instead of just once.The image projected by the electrically controlled optical elements isonly half as bright when the ferro-electric switchable half wave plateis “off” versus “on”, however, because light rays encounter the halfsilvered mirror twice when the ferro-electric switchable half wave plateis “off” instead of just once when it is “on”.

It is easy to appreciate how it is possible to stack together severalindividual electrically controlled optical elements to create an opticaldevice with multiple possible effective focal lengths and viewingangles. The ferro-electric switchable half wave plate in eachelectrically controlled optical element stack can be individually turned“on” and “off”; therefore, an effective focal length for the design canrange from a short distance, when all the ferro-electric switchable halfwave plates are “on”, to a long distance, when the ferro-electricswitchable half wave plates are “off.” In-between effective focallengths are possible by turning some of the ferro-electric switchablehalf wave plates “on” and some “off”. The distance between the halfsilvered mirror and the reflective polarizer could be tailored in eachelectrically controlled optical element stack in order to meet designcriteria. The polarizers in each individual electrically controlledoptical stack must be designed to transmit the polarized light thatexits the previous electrically controlled optical element stack.

Those of skill in the art will recognize that it is possible tofabricate electrically controlled optical elements that project asmaller image or create an image with a smaller field of view when theferro-electric switchable half wave plate is “on” without departing fromthe spirit of the invention. This can be done, for example, by replacinglens 30 with a diverging lens. The rest of the components of such asystem would operate in the same manner as described in the precedingparagraphs.

Application to Zoom Lenses

It is also advantageous to use electrically controlled optical elementsin applications such as zoom lenses. FIGS. 5a and 5b schematicallyillustrate in cross sectional view the optical elements of a prior artzoom lens in a first and second zoom state, respectively. The operationof zoom lens 50 is familiar to those of skill in the art. In FIGS. 5aand 5b , light rays 60 from an object 62 are focused by the zoom lens onto an internal surface 64. Surface 64 may be, for example, the surfaceof a charge coupled device, light sensitive film, or the like. In orderto change the magnification of the image of object 62 on surface 64, theeffective focal length of the lens is changed. This is executed bymoving lens group 54 relative to lens 52 and lens group 56. A first zoomstate is illustrated in FIG. 5a , with lens group 54 is located adistance g from lens 52 (for example, 2.6 inches) and a distance h fromlens group 56 (for example, 1.5 inches), resulting in an effective focallength of, for example, two inches. A second zoom state is illustratedin FIG. 5b ; lens group 54 is moved to a distance i from lens 52 (forexample, 2.9 inches) and a distance j from lens 56 (for example, 1.2inches) resulting in an effective focal length of, for example, 5.9inches. In typical zoom lenses, the manipulation of lens group 54 isperformed using mechanical adjusters or a motor. These adjusters take upspace, which is often undesirable. Also, while the overall length ofzoom lens 50, represented by distance k (for example, 12.5 inches), doesnot change when moving lens group 54, in many zoom lenses the overalllength does not remain fixed while altering the lens' focal length. Thiscan be problematic if a static overall lens length is desired.

Alternately, electrically controlled optical elements may be usedinstead of mechanical adjusters to alter the effective distances betweenthe lenses and lens groups. FIGS. 6a and 6b schematically illustrate incross sectional view a zoom lens that employs electrically controlledoptical elements in a first and second zoom state, respectively. Theoptical elements of zoom lens 70 remain unchanged from zoom lens 50except for the addition of electrically controlled optical elementgroups 72 and 74. Electrically controlled optical element group 72 islocated between lens 52 and lens group 54, and electrically controlledoptical element group 74 is located between lens group 54 and lens group56. Lens 52 and lens group 54 are physically separated by distance 1(for example, 2.6 inches), and lens group 54 and lens group 56 arephysically separated by distance m (for example, 1.2 inches). The lengthof the lens 70 stays the same constant distance k (for example, 12.5inches). Unlike with zoom lens 50, these physical distances do notchange as the focal length of lens 70 changes. In this particularexample, the ferro-electric switchable half wave plates always operatein opposite states; i.e., the ferro-electric switchable half wave platein electrically controlled optical element group 72 is on when theferro-electric switchable half wave plate in electrically controlledoptical element group 74 is off, and vice versa. A first zoom state isillustrated in FIG. 6a ; the ferro-electric switchable half wave platein electrically controlled optical element group 72 is “on”, while theferro-electric switchable half wave plate in electrically controlledoptical elements 74 is “off.” Therefore, the light path between lens 52and lens group 54 is unaffected, and light traveling from lens 52 tolens group 54 must travel a distance 1. The light traveling between lensgroup 54 and lens group 56 travels three times between the half silveredmirror and the reflective polarizer in electrically controlled opticalelement group 74. This results in an effective light path length that islonger than distance m (for example, the effective light path lengthbetween lens group 54 and lens group 56 may be 1.5 inches). The secondzoom state is illustrated in FIG. 6b ; the ferro-electric switchablehalf wave plate in electrically controlled optical element group 72 is“off”, while the ferro-electric switchable half wave plate inelectrically controlled optical elements 74 is “on.” The light travelingbetween lens 52 and lens group 54 travels three times between the halfsilvered mirror and the reflective polarizer in electrically controlledoptical element group 72. This results in an effective light path lengththat is longer than distance 1 (for example, the effective light pathlength between lens 52 and lens group 54 may be 2.9 inches). The lightpath between lens group 54 and lens group 56 is unaffected, and lighttraveling from lens group 54 to lens group 56 must travel a distance m.

Those of skill in the art will recognize that it is possible to use moreor less electrically controlled optical element groups in a zoom lens toachieve a similar result without departing from the spirit of theinvention. Those of skill in the art will recognize that differentconfigurations also exist, such as ones that do not require eachelectrically controlled optical element group to operate in oppositestates.

While the foregoing invention is described for use in applications suchas viewfinders and zoom lenses, the invention finds relevance withoutlimitation in a wide range of applications. Electrically controlledoptical elements may be used, for example, to create diffuser elementswith variable diffuser angles, variable-magnification compactmicroscopes, and the like.

We claim:
 1. An electrically controlled optical system for controlling afocal length of an optical system in a light path between a lens and adisplay producing polarized light in a head mounted display comprising:a quarter wave plate in the light path between the lens and the display;a partially silvered mirror in the light path between the quarter waveplate and the display; a switchable wave plate having two or morestates, the switchable wave plate in the light path between thepartially silvered mirror and the display; electrical means forcontrolling the state of the switchable wave plate operably connected tothe switchable wave plate; an airspace between the partially silveredmirror and the switchable wave plate; and a polarization dependentmirror in the light path between the switchable wave plate and the lens.2. The electrically controlled optical system of claim 1 wherein theairspace is located between the switchable wave plate and thepolarization dependent mirror.
 3. The electrically controlled opticalsystem of claim 2 further comprising: a polarizer in the light pathbetween the polarization dependent mirror and the display.
 4. Theelectrically controlled optical system of claim 1 further comprising: apolarizer in the light path between the polarization dependent mirrorand the display.
 5. An electrically controlled optical system forcontrolling a focal length of an optical system in a light path betweena lens and a display producing polarized light in a near-to-eye displaycomprising: a quarter wave plate in the light path between the lens andthe display; a partially silvered mirror in the light path between thequarter wave plate and the display; a switchable wave plate having twoor more states, the switchable wave plate in the light path between thepartially silvered mirror and the display; electrical means forcontrolling the state of the switchable wave plate operably connected tothe switchable wave plate; an airspace between the partially silveredmirror and the switchable wave plate; and a polarization dependentmirror in the light path between the switchable wave plate and the lens.6. The electrically controlled optical system of claim 5 wherein theairspace is located between the switchable wave plate and thepolarization dependent mirror.
 7. The electrically controlled opticalsystem of claim 6 further comprising: a polarizer in the light pathbetween the polarization dependent mirror and the display.
 8. Theelectrically controlled optical system of claim 5 further comprising: apolarizer in the light path between the polarization dependent mirrorand the display.
 9. An electrically controlled optical system forcontrolling a focal length of an optical system in a light path betweena lens and a display producing polarized light comprising: a quarterwave plate in the light path between the lens and the display; apartially silvered mirror in the light path between the quarter waveplate and the display; a switchable wave plate having two or morestates, the switchable wave plate in the light path between thepartially silvered mirror and the display; electrical means forcontrolling the state of the switchable wave plate operably connected tothe switchable wave plate; an airspace between the partially silveredmirror and the switchable wave plate; and a polarization dependentmirror in the light path between the switchable wave plate and the lens.10. The electrically controlled optical system of claim 9 wherein theairspace is located between the switchable wave plate and thepolarization dependent mirror.
 11. The electrically controlled opticalsystem of claim 10 further comprising: a polarizer in the light pathbetween the polarization dependent mirror and the display.
 12. Theelectrically controlled optical system of claim 9 further comprising: apolarizer in the light path between the polarization dependent mirrorand the display.